The present invention relates to a method and apparatus for use in the detection of liquids, particularly hydrophobic liquids such as oils. It may be used for monitoring for leakage. A preferred type of embodiment employs an optical method that is capable of detecting and preferably identifying hydrocarbon liquids such as mineral or synthetic oils, petroleum, diesel, insulating oils etc. emanating from a leaking vessel, container or storage device.
A number of vessels are routinely used to carry or hold hydrocarbon solvents such as oils in a wide variety of applications. Examples include oil filled underground power cables, underground petroleum storage tanks, above surface oil containers for industrial or domestic heating, oil filled power transformers and equipment etc. Release of the container""s contents, deliberate or accidental or via corrosion over time will have economic and environmental consequences. The routine monitoring of the release of oil/solvents from these containers can be an arduous task because of the myriad of locations, distribution and varieties of such devices. A more convenient method would be one where a sensor device is placed at each container location and was able to perform continues or periodic monitoring as required. Ideally, the sensor should be able to detect any oil/solvent spillage as soon as it occurs in order that remedial actions can be made to minimise any ongoing loss into the environment. Such a device is referred to here as an in-situ sensor or monitor. Ideally, a remotely controlled sensor device that could automatically warn of oil/solvent leakage is preferred especially where container vessels or locations are difficult to access, are widely distributed or where checks for leakage are made infrequently. Examples of such situations are discussed below.
The continual supply of electrical power throughout a country relies on the integrity of underground and overhead power cables. Ease of maintenance requires that most power cables are run above ground, but where this is not possible, such as in cities, the power lines are buried some meters underground. At the operating voltages of 132 kV and above, many of the cables in service are of the oil filled type. Oil-filled cables are normally laid in sections of between 200-400 meters, which are then joined together in specially constructed joint bays. The cables and joints are then encased in a special backfill material such as speciality grade sands or cement-bound-sand (CBS).
Voids in the cable insulation can result in partial discharge activity and ultimately electrical breakdown of the cable. In an oil filled cable the oil, if maintained under sufficient pressure, prevents the formation of gaseous voids. The hydraulic system is designed to be maintained at a positive pressure at the highest points on the route profile and for this maximum static pressure at the lower points on the profile can be up to 5.25 bar.
Problems arise with this type of cable when leaks appear in the pressure retaining metal jacket. Where there is a leak, the cable must be switched out if adequate pressure cannot be maintained in order to prevent the risk of electrical breakdown of the cable. The oil used in new cable installations is a synthetic mixture of alkylated benzenes, with the greatest component being dodecylbenzene (DDB). (Older cable installations employ mineral oil for insulation, but this is gradually being phased out.) Although it has not been shown to be directly carcinogenic to humans, it is of a class of chemicals (substituted benzenes) some of whom do have toxic properties, so there are environmental implications associated with leakage of this oil.
Due to the nature of the cable, leaks most often occur where two cable ends are joined, in specially constructed joint bays. Leaks in the body of the cable are much rarer, and are usually only caused by the over-zealous use of earth digging machinery, and so are usually located immediately. Nevertheless, should a leak occur it would be detected by a fall in operating pressure over a period of time. Then the problem lies in locating in which of the many joint bays along the length of the cable the leak has sprung. Prior to this invention, detection relied on hydraulic bridge techniques, which are both time consuming and unreliable. A much-preferred method would be to install in each joint bay a device capable of providing immediate notification of a leak condition and thus ensure swift remedial measures, avoid the risk of excavating a healthy bay and minimise any disruption to power supplies.
The burial of tanks containing hydrocarbon liquids has been a method of storage around the world. One of the main reasons that this method is employed is for the reduction in the risk of fire and explosion that is afforded. When sited underground the tank is protected from damage by the myriad of possible causes, and will also save on space. However, placing tanks under the ground has its own hazards. The particular problem with underground tank storage is one of tank corrosion. Whereas above ground tanks are easily inspected, underground tanks by the nature of their position are a more difficult monitoring challenge. The stability of the soil is not easily assessed, any leaks that may be occurring may continue for months or even years, and even a small leak of one drop per second may result in a loss to the soil of 400 liters per annum. Awareness of the problems has been increasing; in Britain, particularly with the Environmental Protection Act of 1990 and the Environment Act of 1995 emphasising the polluter pays principle, and increased concern over water supplies. In the USA, awareness and concern are particularly high, especially in some areas where dependence on groundwater is high. The Environmental Protection Agency estimates that 41,600,000 liters of petrol alone may be leaking from underground storage tanks every year.
A preferred route to monitoring possible leaks from underground tanks would be to install an in-situ sensor device that was buried near the tank and capable of sensing whether or not a leak had occurred, thus allowing the user to determine the integrity of the vessel on a continuous basis.
One design for such a sensor (TraceTek from Raychem, USA) (see http://www.raychem.com/products/chemelex/technolocy.htm for details.) involves the two poles of an electrical switch being separated by a degradable polymer. Not only is this system expensive and difficult to install it can also produce false indications where there are low level background traces of oil, as this tends to degrade the polymer over extended periods of time.
Other methods of leak detection involve hydraulic bridge techniques. This system requires very small pressure differences to be measured and these measurements can be difficult where there are transient pressure variations and/or where cable records are unreliable and/or where there are localised thermal conditions owing for example to another heat source.
According to the present invention in a first aspect there is provided a method of monitoring for the presence of liquid at a site comprising: locating at said site a sensor assembly comprising a radiation source and a radiation detector and/or analyser arranged to detect and/or analyse radiation which results from the emission of radiation by the source; causing the radiation source to irradiate a sensing location; and employing said detector/analyser to receive radiation, the arrangement being such that the nature and/or amount of radiation received by the detector/analyser is affected by the presence of liquid at the sensing location. The liquid may be a hydrophobic liquid such as oil. The sensor assembly may include a hydrophobic membrane or other element which preferentially takes up hydrophobic liquid. This affects its optical properties, e.g. reflectance of light at a membrane/glass interface. The element may be or include a fluorocarbon, e.g. polyvinylidene fluoride.
Radiation from the source interacts with the liquid in the sensing location, e.g. by one or more of reflection, absorption, transmission, scattering and fluorescence. Radiation resulting from the interaction is detected and/or analysed by the detector/analyser.
In a second aspect the invention provides an assembly comprising a vessel containing a liquid and a sensor assembly located at a site potentially contaminated by liquid leaking from the vessel and adapted to carry out the method as defined above.
In a third aspect the invention provides a sensor assembly for use in monitoring for the presence of hydrophobic liquid at a site, said assembly comprising: a hydrophobic element which is disposed so that in use it is exposed to the environment at the site and which is adapted to take up hydrophobic liquid; a radiation source arranged to irradiate at least a portion of the hydrophobic element; and a radiation detector and/or analyser arranged to receive radiation resulting from the interaction of the source""s radiation with the hydrophobic element.
In a preferred type of embodiment the invention provides an in-situ device for the detection and identification of oil or other hydrocarbon products that have leaked from vessels such as underground power cables or petrol storage tanks.
Detection of oil may be achieved by measuring the intensity of light reflected or emitted from a hydrophobic membrane at an optical window in contact with the external environment, when oil is present in the environment, it is absorbed into the hydrophobic membrane causing a change in the intensity of the reflected beam. The membrane can be one from the fluorocarbon range of membrane materials such as polyvinylidene fluoride. Identification of the oil is provided by measuring the spectral properties of the reflected beam or the spectral properties of light emanating from oil absorbed in the membrane.
In a preferred embodiment, the sensor has been designed for specific application to detecting oil leakage from underground power cables although it has clear application in other situations where oil or other hydrocarbons may leak from a vessel located underground, above ground or in water.
There may be a plurality of sensors for installation at different locations around a potential source of liquid. They may be connected via waveguides (such as fiber optics) to a detector/analyser. There may be a xe2x80x9cmultiplexing unitxe2x80x9d such that the detector/analyser is connectable to one sensor at a time, the connected sensor being selectable and/or determined by programmed switching.
The invention can be left to operate in-situ at the monitoring site enabling convenient continuous or periodic monitoring of the environment. Using telecommunications methods known to those skilled in the art, it is possible to transfer data from the monitoring site to a remote destination. The invention has several applications where monitoring oil or hydrocarbon leaks is required or preferred. Examples are shown in the technical description.
In the preferred embodiment, the device can be buried in sand or soil or immersed in water that surrounds an oil carrying vessel or container. If oil leaks from the vessel and contacts the sensor, it will be detected as a change in signal intensity or spectral characteristics. Such a system can therefore be used as an in-situ monitoring device that is triggered when a leak has occurred.